Carbon fibers have been used in a wide variety of applications from aircraft to building materials. If their productivity is improved and their cost is lowered more and more, they can be materials in place of stainless steel plates also in automobile body and the like. At present, carbon fibers are mainly produced using polyacrylonitrile (PAN) fibers and pitch fibers as raw materials (carbon fiber precursor fibers).
These carbon fiber precursor fibers, however, need a pre-treatment called an infusibilization treatment prior to carbonization, and this treatment is a major barrier to reduction in cost and energy required for their production, and to increase in productivity.
Specifically, since PAN fibers and pitch fibers are fused in the course of a carbonization treatment (a high-temperature thermal treatment at 1,000° C. or higher) and cannot maintain their fiber shapes, they are changed to infusible, flame-resistant fibers by an air oxidization treatment called an infusibilization treatment and then are subjected to carbonization to obtain carbon fibers. In this infusibilization treatment, it is necessary to uniformly control oxidation reaction and also strictly manage temperature conditions for suppressing thermal runaway due to exothermic reaction, and moreover its treatment time is long (about 30 minutes to about 1 hour).
Meanwhile, some kinds of heat-resistant aromatic polymers (e.g., aramid fibers and phenol resin fibers) have such properties that they are carbonized without being fused, and thus it is possible to obtain carbon fibers only by forming such polymers into fibers and subjecting the resultant fibers to a high-temperature thermal treatment.
Although aramid fibers and phenol resin fibers are carbonized while maintaining their fiber shape, they have problems that their mechanical strength is poor.
That is, when only carbonization is performed while shapes are being maintained, sufficient mechanical properties (e.g., strength and elasticity) required for carbon fiber products are not developed, and thus there is still a need to develop new materials realizing sufficient mechanical properties.
Here, the present inventors previously found out a graphite film containing a heterocyclic polymer obtained through condensation between an aromatic tetracarboxylic acid and an aromatic tetraamine (see PTL 1).
However, when crystallization excessively high in two-dimensional (layer-form) orientation occurs like in a graphite film, cracks of fibers occur due to delamination in a parallel direction to graphite crystal layers bonded only via intermolecular force, and strength as fibers is problematically very weak.